Tetartanomaly
A type of blue-yellow Anomalous Trichromacy that has many theoretical mechanisms, but none of which have ever been detected in humans. It would have the same similarities to Tritanomaly that Deuteranomaly has to Protanomaly.
Tetartanomaly would require a ‘weakening’ of the yellow side of the blue-yellow Opponent Channel. Because the yellow excitation is equal to the sum of excitations of the L- and M-Cones (together representing yellow), some of these theoretical mechanisms are:
- Looking through a blue tinted lens that blocks yellow light, similar to the cataract-related cause of Cyanopsia. This would be the opposite of one common cause of mild Tritanomaly, which is age-related yellowing of the cornea, which is akin to looking through a yellow lens.
- A decrease of the sensitivity of either the L- or M-Cones would result in a tetartanomaly, but would also lead to a red-green anomaly. For the tetartanomaly to exist without red-green anomaly, both cones would have to decrease in sensitivity simultaneously and to a similar degree. There is no known mechanism for decreasing the sensitivity of the L- or M-Opsin.
- Significantly changing the Retinal Mosaic by replacing the majority of L- and M- Cones with S-Cones would introduce a Tetartanomaly. Similarly, if the cones lost to Protanopia and Deuteranopia were replaced with S-Cones, this would cause a slight Tetartanomaly in addition to the stronger red-green effect. However, a loss of M-Cones is generally mostly replaced with L-Cones and vice versa (see Cone Mosaic).
- A decrease in the M-Opsin’s Peak Wavelength would cause a very slight Tetartanomaly, but would also lead to a much more significant improvement to the Red-Green Opponent Channel. There is also no known allele for an M*-Opsin with a lower Peak Wavelength.
- A decrease in the Peak Wavelength of both M- and L-Opsins could introduce a Tetartanomaly while minimizing the effect on the red-green channel (though also decreasing the size of the Visible Spectrum).
Like Tetartanopia, there are also theoretical mechanisms that may exist further down the Phototransduction Pathway (Neurological CVD).
If hypothetically human Color Vision consisted of 4 cones, where the 4th cone is introduced with Peak Sensitivity at 545nm, and replaced the L- and M-Cones in the blue-yellow Opponent Channel “calculation”, the resulting vision would be nearly identical to the current Standard Observer. However, if this 4th “Yellow” Cone were then to decrease in sensitivity or be shifted towards the S-Cone in the Absorption Spectrum, then the individual would experience Tetartanomaly without an effect on the Red-Green Opponent Channel.